Sect. I.] 



ASTRONOMY, 







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ddronomical Geography. 



10. The intelUsent navio;ator. on arriving at any port 

 wliicli has not before been visited, or whose position is not 

 very well settled, ought to consider it his first duty to 



determine with all the accuracy in his power tiie latitude 

 and longitude of the port. Supposing him to have deter 

 mined by the usual nautical methods the approximate 



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latitude, longitude, and error of chronometer, the best 

 metliod of determining the latitude will be to find the 

 chronometer-time at which the sun or any bright stars of 

 the Nautical Almanack hst will pass the meridian, and to 

 observe the double altitude of any such object by reflection 

 in a mercurial horizon, several times, as near as possibh? 



to the time of the meridian passage. If the place is in 



the northern hemisphere, the observation of the double 

 altitude of the pole-star may be made at any time when it 

 is visible : convenient tables for the reduction are given iu 

 the Nautical Almanack. For these and other observa- 

 tions the navigator ought to be provided with a proper 

 trough and a store of mercury. For determining the 

 longitude, there is probably no method superior to that of 

 lunar distances (the exactness of which will be increased 

 if the se2Llant or reflecting circle be mounted on a stand), 

 unless the stay at the port is so long that transits of the 

 moon can be observed. In any case, if there be a transit- 

 instrument in the ship, it ought to be mounted on shore a 

 soon as possible. The instrument ought, on the first 





evening, to be got very nearly into a meridional position, 



and then a mark should be set up, and the instrument 

 hould always be adjusted to that same mark (even 



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